A light emitting diode (LED) is composed of an epitaxial structure such as a homo-structure, a single hetero-structure, a double hetero-structure or a multiple quantum well. The LED with a p-n junction interface that can emit light with various wavelengths has several beneficial characteristics, including low electrical power consumption, low heat generation, long operational life, small volume, good impact resistance, fast response and excellent stability. These characteristics have made the LED popular for integration into electrical appliances and electronic devices as a light source.
Typically, an LED is composed of an epitaxial structure with a substrate, an n-type cladding layer formed over the substrate, a p-type cladding layer and an active layer formed between the n-type cladding layer and the p-type cladding layer. Light is emitted as current flows through the epitaxial structure. The light wavelength can be altered by varying the composition of the epitaxial structure material.
In general, the light output of an LED depends on the quantum efficiency of the active layer and the light extraction efficiency. The higher the quantum efficiency of the active layer, the higher the light output of the light-emitting diode. Generally, improving the quality of epitaxial structure and the structural design of the active layer increases the quantum efficiency of the active layer. In addition, as the light extraction efficiency increases, the light output of the light-emitting diode is enhanced. In order to improve the light extraction efficiency, efforts are made to overcome the significant photon loss resulting from total reflection inside the light-emitting diode after emission from the active layer.
When the light generated by the active layer of the conventional LED is emitted downward to a GaAs substrate for example, the light will be absorbed by the GaAs substrate since the GaAs substrate has a smaller energy gap. Accordingly, the light-output performance of the LED will be greatly reduced.
There are some conventional LED technologies that have been disclosed in order to avoid the absorption of light by the substrate. However, these conventional technologies still have some disadvantages and limitations. For example, Sugawara et al. disclosed a method, which was published in Appl. Phys Lett. Vol. 61, 1775-1777 (1992), that added a distributed bragg reflector (DBR) layer on the GaAs substrate so as to reflect the light emitted downward to the GaAs substrate and to decrease the light absorbed by the GaAs substrate. However, because the DBR layer only reflects light that is of near normal incidence to the GaAs substrate, the efficiency is not very great.
Kish et al. disclosed a wafer-bonded transparent-substrate (TS) (AlxGa1-x)0.5In0.5P/GaP light emitting diode [Appl. Phys Lett. Vol. 64, No. 21, 2839 (1994); Very high-efficiency semiconductor wafer-bonded transparent-substrate (AlxGa1-x)0.5In0.5P/GaP]. This TS AlGaInP LED was fabricated by growing a very thick (about 50 μm) p-type GaP window layer using hydride vapor phase epitaxy (HVPE). Before bonding, the n-type GaAs substrate was selectively removed using chemical mechanical polishing and etching techniques. The exposed n-type (AlxGa1-x)0.5In0.5P cladding layers are subsequently wafer-bonded to 8-10 mil thick n-type GaP substrate. The resulting TS AlGaInP LED exhibits a two fold improvement in light output compared to absorbing substrate (AS) AlGaInP LED. However, the fabrication process of TS AlGaInP LED is too complicated. Therefore, it is difficult to manufacture these TS AlGaInP LEDs in high yield and low cost.
Horng et al. reported a mirror-substrate (MS) AlGaInP/metal/SiO2/Si LED fabricated by wafer-fused technology [Appl. Phys Lett. Vol. 75, No. 20, 3054 (1999); AlGaInP light-emitting diodes with mirror substrates fabricated by wafer bonding]. They used the AuBe/Au as the adhesive to bond the Si substrate and LED epilayers. However, the luminous intensity of these MS AlGaInP LEDs is about 90 mcd with 20 mA injection current and is still 40% lower than the luminous intensity of TS AlGaInP LED.